Exhaust gas aftertreatment systems and methods are known for, alone or in combination, regenerating one or more exhaust aftertreatment devices, including regenerating a particulate filter, for example a diesel particulate filter, desulfating a lean NOx trap, and desulfating or regenerating a catalyst, in embodiments a selective catalytic reduction catalyst (SCR catalyst) or a urea-selective catalytic reduction catalyst, located downstream of an internal combustion engine, in embodiments, a gasoline engine or a diesel engine.
Diesel engines are operated at higher than stoichiometric air to fuel mass ratios for improved fuel economy. Such lean-burning engines produce a hot exhaust with a relatively high content of oxygen and nitrogen oxides (NOx). The temperature of the exhaust from a warmed up diesel engine is typically in the range of about 200° C. to about 400° C. and has a representative composition, by volume, of about 10% to about 17% oxygen, about 3% carbon dioxide, about 0.1% carbon monoxide, about 180 parts per million (ppm) hydrocarbons, about 235 ppm NOx and the balance nitrogen and water.
NOx gases, typically comprising nitric oxide (NO) and nitrogen dioxide (NO2), are difficult to reduce to nitrogen (N2) because of the high oxygen (O2) content in the hot exhaust stream. Existing NOx reduction technologies, such as lean NOx trap technology and urea/selective catalytic reduction (SCR) technology, can provide some suitable NOx reduction performance under certain conditions.
Particulate filters, for example diesel particulate filters or gasoline particulate filters, trap soot in the exhaust and need to be regenerated periodically. Regeneration of the diesel particulate filter comprises elevating the temperature of the filter to a temperature sufficient to burn off the soot, typically from about 600° C. to about 700° C., and maintaining that temperature during the cleaning cycle.
A lean NOx trap stores NOx emissions during fuel lean operations and converts the stored NOx, during fuel rich conditions, to nitrogen and water. The lean NOx trap has limited storage capacity and must be regenerated with a fuel rich reducing “pulse” as it nears capacity. It is desirable to control the efficiency of the regeneration event of the lean NOx trap to provide optimum emission control and minimum fuel consumption.
Sulfur in the exhaust gas stream stored on the lean NOx catalyst has a detrimental effect on the lean NOx catalyst ability to reduce NOx emissions. Therefore, the lean NOx trap must be desulfated periodically. Sulfur is removed by providing a rich reducing “pulse” air fuel ratio while elevating the temperature of the lean NOx trap to a temperature of from about 700° C. to about 800° C.
The elevated temperatures are typically achieved through the use of an oxidation catalyst or in the case of a diesel engine a diesel oxidation catalyst (DOC), which generates an exothermal reaction with raw hydrocarbons that are injected into the exhaust stream, such as by in-cylinder injection or external injection directly into the exhaust stream. Temperature control of the oxidation catalyst can be problematic due to the large thermal mass associated with the catalyst as well as the transport delays between the point of injection and the temperature sensor location. Methods that have been used for controlling the temperature include use of open loop tables based on engine operating parameters and closed loop temperature control using temperature sensors at various locations in the exhaust stream. Closed loop temperature control of the oxidation catalyst using temperature sensors can be limited by the slow response of the system. Engine out particulate matter maps, models or particulate matter sensors can also be used.